Vibration and Radiated Noise of a Small Ship Ignacy Gloza
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VIBRATION AND RADIATED NOISE OF A SMALL SHIP IGNACY GLOZA Polish Naval Academy Śmidowicza, 81 - 919 Gdynia, Poland [email protected] Extensive measurements “in situ” both of the sound intensity and the vibration were made of M25, a small ship (length 8 m , beam 4 m, displacement 2 tons) powered by 20 kW a direct-drive low-speed diesel engine. A small ship creates a series of harmonics which amplitudes and frequencies are connected with ship speed. The underwater sound measurement was performed for anchored condition. In this paper, two different methods of measurement were used, which provide complementary information. A static method to measure noise from an anchored ship was used when only the main engine was running. In addition to the radiated noise measurements, vibration measurements were conducted aboard this ship. The coherence function was performed to associate each component of underwater noise with the vibrating part of the engine which generates it. The calculation of the sound intensity was made to locate the main source of noise on board. Underwater noise from small ships elevates the natural ambient by 10 – 20 dB in many area; the effects of this noise on the biological environment have been rarely reported. INTRODUCTION Noise radiated by ships into the water environment is an important contribution to the ocean ambient noise. Therefore controlling acoustic signature on vessels is now a major consideration for researchers, naval architects and operators. Noise is not a major problem for all vessels. For small ships the problem is caused by closely packed high powered equipment, confined in a small metal or plastic vessel. Shipboard noise problems are generally created by poor or improper ship acoustical design. The noisiest piece of equipment on any ship is usually a diesel engine. As a reciprocating machine, the diesel is very loud and also creates a great deal of vibration. Donald Ross studied the underwater noise of big commercial ships and trends in ship sizes and powering. He also wrote a book that described the fundamental general features of surface ship noise [1]. Many European researchers in France, Germany, Norway, Sweden Finland and Poland have also contributed to ship noise and propeller investigation. In recent years sophisticated numerical and experimental research on propeller cavitation has been conducted [2]. A lot of the older published ship underwater noise measurements were made with third- octave bandwidth analysis, which is too wide for separation of the individual spectral components of ship radiation. The data were nearly always acquired in shallow water, so the levels may not be well representative of free-field values, especially at low frequencies. In the 1970 S the US navy initiated a new practical and theoretical program for accurate narrow-band measurements of underwater ship noise [5]. Because of to the high cost of ship time as well as measurement facilities, it is perhaps not surprising that few detailed measurements of merchant ships and warships are available. 1. METHODS OF MEASUREMENTS Static measurement of a small surface ship was conducted while she was moored to buoys in the centre of the range, allowing investigation of the contribution from individual machines and machinery systems to the noise signature. Within the range terminal, hydrophone and vibration signals, either recorded or direct, were processed by the noise analysis of data from all aspects covering frequencies 1 Hz to 10 kHz. In our case the first step was to determine the characteristic frequencies of the main engine, by analyzing the spectrum of engine vibrations. The second step was to identify, in the underwater environment, the underwater noise coming from the ship 2 m below the sea surface. The last step consisted in measuring the sound intensity level around the ship. It allowed to determine the location of machinery which radiated the highest level of noise. These measurements were carried out in the Polish Navy Test and Evaluation Acoustic Ranges in Gdynia, that is located in the southern part of the Baltic Sea. The basin was well protected from wind and waves, but the weather was not specially good, so the ambient noise level was average. It was a windy and raining day. During the ship measurements, the mean wave hight was less than 1 m, with wind speeds less than 6 m/s. The bottom depth was 6m. Measurements were made with an 8 meter long ship – with beam 3m – call signed M25. Its main engine was the only working machinery on board. The ship was in the middle of the basin, bound to the ground by three long hawsers, the propeller was stopped and only the main engine was running. At the time of the hydroacoustical measurements in 2007, M25 was rather old, but her engine, hull and shaft were observed to be in excellent condition. No fouling or damage was evident on the hull or engine. This ship was powered by a four- stroke four-cylinder diesel engine, that vibrated with firing rate equal to 6.2 Hz. In order to measure vibrations and propagation of the waves through the ship, three piezoelectric accelerometers were used. The first one was fixed by the magnetic connection directly to the engine. The two others were stuck to the hull: one in the middle of the boat and the other one was located on the bow. They were all connected to the conditioning amplifiers which were able to amplify low signals coming from the piezoelectric accelerometers. The pressure signals using two hydrophones were measured, they were connected to a wide range measuring amplifiers. The hydrophones were joined together at a distance of 14cm which enables measurements of the sound intensity. Fixed at the extremity of a 3 meter long boom, they were moved around the ship, 2 meters in depth, to carry out sixteen measurement points: seven on port side, one in the bow, seven on the starboard side and one astern. Two railings were attached on the boards to have an accurate position of the hydrophones in relation to the boat. Figure 1 shows the locations of the different measuring positions. For measurements 1 to 8 accelerometers were fixed on the port side, and for measurements 9 to 16, they were connected on the starboard side. Fig.1 Scheme of the small ship with two frames and different detectors The methods of measurements the rotational and translation components of the vibration or structure borne sound levels on a stationary vessel and moving ship are a mixture of analog and digital techniques. The resulting spectra were made digitally both by a Brüel & Kjær PULSE analyzer and a computer. A simultaneous on-board vibration monitoring system provided additional measurements of tonals from inside our boat, because an accelerometer was mounted on the diesel engine. 2. RESULTS OF RESEARCH A general quantitative description of the vibration and the ship’s radiated noise should be provided by power spectral analysis. To understand how this ship generates noise, we should first analyze the spectrum of vibrations from the main engine. It is shown below on figure 2: Fig.2 Spectrum of vibrations from the main engine It was done to identify the engine parts responsible for the most important vibrations. In fact, mechanical unbalance, impact, friction, and pressure fluctuation generate vibratory forces. The dominant noise of a diesel engine is normally the piston slap. It is caused by the impact of the piston on the cylinder wall [6]. The table below describes the characteristic vibration frequencies of the engine. Tab.1 Table of main vibration frequencies of a diesel engine Vibration frequencies Cause of vibrations ⋅ nk f = s Cylinder firing rate cfr 60.2 kn⋅ f = s Crankshaft c 60 kz⋅⋅⋅ n z f = psz Engine valves v m⋅60 kz⋅ ⋅ n f = ps Piston slap ps 60 kbzn⋅ ⋅ f = ps⋅ Piston rings pr 60 where k is the number of the harmonic (it is a whole number), ns is the rotational speed of the engine in rpm, zp is the number of pistons in the engine, zz is the number of valves for one piston, b is the number of piston rings for one piston, m indicates if the motor is a two or four-stroke engine. During the measurements, the engine should run at about 750 rpm or 12.5 Hz, because the engine spectrum contains a peak at 12.3 Hz, it means that the accurate run of the engine was ns =738 rpm (12.3 · 60). We could then obtain the theoretical vibrating frequencies due to the engine parts: Tab.2 Table of fundamental vibration frequencies for the main engine Fundamental frequency Cause of vibrations fcfr = 6.2Hz Cylinder firing rate fc = 12.3 Hz Crankshaft fv = 24.6 Hz Engine valves fps = 49.3 Hz Piston slap fpr = 98.6 Hz Piston rings Lots of harmonics are associated with those frequencies. We cannot know exactly the vibration level of each harmonic, but the scheme below summarizes the contribution of each source in the spectrum. Fig.3 Scheme describing the contribution of sources for some vibration frequencies. The bandwidth is 0.1 Hz The fundamental frequency of cylinder firing rate is associated with a mess of harmonics. But the corresponding level is not the highest. We have to focus on the highest levels, which are normally 24.6 Hz and 49.3 Hz. You can notice a peak at 50 Hz in figure 4: Fig.4 Zoom of figure 3 (frequency around 50 Hz). The bandwidth is 0.05 Hz This peak is connected with the alternating current which powers all electric equipment. As it was very difficult to connect the grounds of all electric equipments together, a DC component of 50 Hz appeared during the measurements.